Borehole casing



March 31, 1664 E, ORTLOFF 3,126,959

BOREHOLE CASING Filed Feb. 26. 1960' John E. Ortloff Inventor By 9m) M Attorney United States Patent 3,126,959 BOREHQLE Q'JASENG John E. @rtloif, Tulsa, @lda, assignor to .liersey Production Research Company, a corporation of Delaware Filed Feb. 26, E69, Ser. No. 11,291 7 Claims. (Cl. ins-33 The present invention relates to methods for installing casing in oil wells, gas wells and similar boreholes drilled into the earth and more particularly relates to a process for applying a plastic material to the wall of such a borehole and thereafter hardening it in situ to form a casing which is resistant to attack by borehole fluids and capable of withstanding relatively high temperatures and pressures. In one embodiment, the invention also relates to a tool which is particularly useful for the in situ formation of casing from plastic materials in wells and similar boreholes.

In order to avoid damage to the walls as a result of pressure and the presence of fluids therein, it is conventional to install casing in oil wells, gas wells and similar boreholes. The casing used for this purpose is generally large diameter, heavy steel pipe containing Welded or threaded joints. It is set into place by lowering it into the borehole and thereafter injecting a slurry of cement into the annular space between the casing and the borehole wall. The cement supports the casing, prevents vertical communication between strata surrounding the borehole, and affords some protection against corrosion on the outside of the casing. This method of casing wells and similar boreholes has been found very satisfactory and is used almost universally. Its chief disadvantage lies in the expense involved. Experience has shown that about 25 percent of the total cost of drilling and completing an average oil well in the United States can be directly attributed to the installation of easing. This is due pri marily to the cost of the steel casing itself. The price of steel oil well casing has advanced continuously in recent years and it is not likely to decrease in the near future.

In order to reduce the over-all cost of drilling and completing wells and similar boreholes, it has been suggested that other materials be substituted for the steel casing conventionally used. Preformed plastic pipe has often been proposed. It has been found, however, that such pipe is generally not competitive with steel casing because of ;.its relatively low strength. The wall thickness necessary to withstand the pressures existing in many wells makes the cost of using such pipe prohibitive. Other materials suggested have likewise been unable to meet the rigid requirements for borehole casing and have proved equally unsatisfactory.

The present invention provides a new and improved method for easing oil wells, gas wells and similar boreholes which has advantages over the conventional method and substitute methods proposed in the past. In accordance with the invention, it has now been found that the application of plastic materials to the walls of wells and similar boreholes under pressure and the subsequent hardening or curing of the plastic under controlled temperature conditions results in a casing that is impervious to fluids normally present in such wells and boreholes, is capable of withstanding relatively high borehole temperatures and pressures, is not subject to corrosion, and is more attractive from an economic standpoint than conventional steel casing. The plastic material applied in this manner invades the borehole wall a short distance and, as it hardens, forms an extremely strong bond with the rock making up the borehole wall. Channels in the wall surface which might otherwise lead to vertical permeability between non-adjacent strata outside the casing are filled "ice by the plastic and hence fluids cannot migrate from one stratum to another. Smoothing of the plastic as it sets in place results in an even inner surface which is free of projections that might interfere with fluid flow or impede the movement of tools in the well or borehole.

Casing formed in situ in accordance with the invention is particularly attractive in that it has much greater resistance to chemical and electrolytic corrosion than does conventional steel casing. It does not permit the buildup of large deposits of scale, parafiin and gypsum deposits on the casing surface. Since the plastic casing formed in situ is bonded directly to the borehole wall, the use of cement is unnecessary. Its use obviates the necessity for handling heavy lengths of pipe and making up hundreds of joints during the casing program. Finally, the plastic casing can be drilled and circulated out of the borehole without difliculty if removal becomes necessary for any reason. These advantages, coupled with a reduction in the over-all cost of casing the well or borehole, make the use of plastic casing formed in situ extremely attractive. A wide variety of plastic materials may be employed in carrying out the process of the invention. In general, the materials suitable for this purpose are polymers which can be solidified to form high molecular weight solids resistant to chemical and thermal degradation under borehole conditions. Such polymers may be of either the thermoplastic or thermosetting type. They may initially be liquid resins or may instead be powdered or granulated solids that can be liquified by the addition of solvents, liquid catalysts or liquid stabilizers to form plastisols, organosols or modified plastisols. The materials used should have heat deformation temperatures as determined by ASTM Test Method D648 above about F., preferably above about 200 F., and should not be seriously affected by weak acids and bases when tested by ASTM Test Method D-543. Many plastic materials possessing these properties are marketed commercially for use in casting and low pressure laminating applications and hence will be familiar to those skilled in the art.

Specific examples of plastic materials which may be utilized for purposes of the invention include acrylic-type resins prepared from methyl methacrylate, ethyl acrylate, n-butyl methacrylate, isobutyl methacrylate, ethyl methacrylate and similar esters, alone or in combination with other monomers; vinyl polymers prepared from vinyl chloride, vinyl acetate, polyvinyl alcohol, polyvinyl acetal, polyvinyl butyral, polyvinyl formal, vinylidene chloride and the like; allyl resins such as allyl diglycol carbonate; epoxy resins prepared from epichloro'nydrin, bisphenol-A and similar diphenols; glyceryl phthalate and similar castable alkyd resins; phenolic resins prepared by the reaction of phenol with formaldehyde, acetaldeyhde, furfural or the like; urea formaldehyde resins; polyester resins prepared by the copolymerization of a dihydric alcohol such as ethylene glycol, an unsaturated dibasic acid such as fumaric acid, and an unsaturated monomer such as styrene or the like; polyurethanes derived from polyisocyanates such as toluene diisocyanate and polyols, including glycols, polyesters and polyethers; silicones produced by the hydrolysis and condensation of organosilanehalide intermediates; and styrene polymer and copolymer casting resins. In addition to the foregoing, other plastic materials marketed commercially for casting purposes may be utilized in carrying out the process of the invention.

Although plastic materials such as those described above are all available in liquid form, their physical and chemical properties vary considerably and hence methods for using them differ somewhat. Some of the materials, methylmethacrylate, for example, are generally supplied as liquid agents containing hydroquinone or a similar inhibitor which must be removed before the material can be solidified. Other materials such as the phenolic resins are liquids to which must be added a catalyst or hardener before they can be solidified. Still others solidify at elevated temperatures without either the removal of an inhibitor or the addition of a catalyst or hardener. The procedure for handling any particular casting material and the catalysts, hardeners, plasticizers, cross-linking agents and other materials suitable for use therewith can readily be determined by reference to the trade literature and hence need not be described in detail for purposes of the present invention.

In general it is preferred to admix powdered fillers with the liquid casting materials before they are used. A variety of fillers may be employed, the material selected depending to some extent upon the properties of the casting plastic. Typical fillers include sand, powdered limestone, asbestos particles, short fibers, mica, chalk, diatomaceous earth, clay and the like. The use of such fillers increases the strength and impact resistance of the solidified plastic; reduces the amount of plastic material required to complete a casing of given size; improves resistance to heat, moisture and chemical attack; and reduces shrinkage of the plastic as it solidifies. Sand is particularly preferred for use as a filler in the in situ formation of borehole casings because of its low cost and ready availability. The filler employed may constitute 50 percent by weight or more of the total casing material. The exact amount used, of course, will depend somewhat upon the properties of the plastic and the filler utilized.

In many cases it will be preferred to add powdered or granulated polymer to the liquid casting material used in order to accelerate the setting of the plastic and reduce the shrinkage which occurs during setting. The use of such powder or granules also tends to reduce the exothermic heat reaction during the setting of some plastics, since the heat of polymerization tends to be counterbalanced by the heat cf solution of the solid polymer. Certain of the casting liquids may also be prepolymerized in order to reduce shrinkage as the resin hardens. The extent to which prepolymerization can be carried out and the conditions necessary will, of course, Vary with different casting liquids.

It will be understood that all of the thermoplastic and thermosetting resins set forth above are not equally satisfactory for purposes of the invention. In general, the thermosetting resins are preferred over the thermoplastic materials because of their higher strength and better resistance to thermal degradation. Many plastics and resins are greatly improved, particularly with respect to their high temperature properties, by polymerization in the presence of gamma radiation. The inclusion of a radioactive source in the well'oore during the in situ formation of casing will in many cases permit the use of plastic materials which would otherwise not be suitable for purposes of the invention because of their low strength and low softening point.

In utilizing polymeric casting materials of the type described above for the in situ formation of borehole casing, the liquid plastic is forced into contact with the borehole wall to form a lining which is supported in place under controlled temperature conditions until it hardens. The pressure necessary to cause the plastic to invade the wall enough to effect a good bond with it will depend largely upon the viscosity of the liquid plastic, the amount and type of filler used in the plastic, and the porosity of the strata surrounding the borehole. In general, pressures between about 100 lbs. per square inch and about 3,000 lbs. per square inch in excess of the formation pressure will be satisfactory. Pressures within this range can readily be obtained with conventional pumping equipment located on the surface and hence the plastic can be injected down the borehole under pressures sufficient to force it into the surrounding strata.

The temperature at which the plastic or resin is applied to the borehole wall will obviously depend upon the thermal characteristics of the material employed. When thermoplastic resins are used, the plastic material will normally be applied to the wall at ambient temperatures and thereafter cured at temperatures ranging up to about 150 F. Thermosetting resins, on the other hand, are normally heated to somewhat higher temperatures, generally ranging to about 200 F. or higher, in order to cure them. Even though certain of the thermosetting materials can be cured at ambient temperatures over extended time periods, it is in most cases preferred to utilize elevated temperatures in order to accelerate the curing process. Elevated temperatures also reduce the viscosity of the liquid plastic and hence improve penetration of the plastic into the borehole wall. Information concerning optimum curing temperatures for specific plastics is readily available from manufacturers catalogs and the trade literature.

The period of time during which the plastic must be supported in place after it has been applied to the bore hole wall is dependent upon the time required to cure it to the point where it is self-supporting. In the practice of the invention, the curing periods may range from a few minutes to several hours. A preferred method for supporting the plastic during this period is to utilize a mandrel or similar device as a casing tool about which the plastic can be applied to the borehole wall under pressure. An electric heater or suitable heating coils can be incorporated into the mandrel for purposes of temperature control. The outer surface of the tool should be polished and will preferably be chrome plated in order to prevent adhesion of the plastic to it. The plastic can be injected into the annular space between the tool and the borehole wall near the upper end of the mandrel and supported in place by the lower end of the tool as it moves upwardly in the borehole. This permits a continuous casing process and greatly accelerates the placement and curing of the plastic. The method of the invention is not limited to the use of such a tool, however, since other apparatus may be employed to apply and cure the material used to form the casing.

The exact nature and objects of the invention can be more fully understood by referring to the following detailed description of the process and preferred apparatus for use therein and to the accompanying drawing illustrating the process and apparatus.

Turning now to the drawing, reference numeral 11 in dicates a well or similar borehole drilled into the earth by conventional means. The diameter of the borehole is not highly critical and may range from a few inches up to a foot or more, depending upon the purpose for which the borehole is to be used. Suspended in borehole 11 by means of a hose or a string of pipe or tubing 12 is cylindrical casing tool 13. The tool is attached at its upper end to the tubing by threads or similar connecting means 14 and contains a passage 15 which communicates with the bore of the tubing. Passage 15 in the upper part of the tool opens into a lower chamber 16. Ports 17 in the wall of chamber 16 permit fluids to flow from the chamher into the annular space between the tool and the borehole wall 18. An annular packer 19 of rubber, asbestos or similar material is carried on the outer surface of the tool about ports 17. It is preferred that an inverted cuptype packer as shown in the drawing be used in order to insure maintenance of the seal between the tool and borehole wall as the tool is moved upwardly in the borehole. The packer will preferably be provided with several lips and be made sumciently long to insure a seal even though the borehole diameter varies considerably. Means for retracting the packer during trips into the borehole may be provided if desired. A preferred type of retractable packer is one actuated by a bimetallic element or similar means responsive to changes in temperature. Heat used to set the plastic can then be employed to expand the packer within the hole.

The lower portion of tool 13 below chamber 16 contains a chamber which is open at the bottom. An electrical heater 21 is positioned in chamber 20 and supported at its upper end therein by shank 22 which is connected to the tool body by threads or similar means 23. The heater is powered by an electrical cable 24 which extends upwardly through the tool and is carried on the outer surface of tubing 12 to a suitable power source outside the borehole. A vent tube extends between chamber 20 and the upper portion of the tool above packer 19. The outer surface of the tool below the packer is polished and is preferably chrome plated in order to reduce the tendency of the plastic to adhere to the tool during use. In general, adhesion of the plastic to the tool does not occur as long as the tool is moved at the proper rate and hence no difficulties are encountered. The overall dimensions of the tool will depend on the borehole diameter and the period required to cure the plastic utilized. In most cases, the outside diameter of the tool below the packer will be from about /2 to about 2 inches less than the diameter of the hole. This permits the formation of ca ing of from about A to about 1 inch thick on the borehole wall. The length of the tool below ports 17 may be relatively short where quiclc curing resins are to be used or may be quite long in the event that materials requiring an extended period of time for curing are to be employed.

The size and capacity of heater 21 will obviously depend upon the thermal characteristics and curing properties of the resin or plastic employed. In tools to be used with thermosetting resins, it is generally preferable that the heater be one in which the temperature varies over the length of the unit. By providing higher temperature coils near the lower end of the tool, the temperature of the resin can be gradually increased as the tool is raised in the borehole. In lieu of a single heater, a series of interconnected heaters of varying capacity may be utilized. Where a thermoplastic material is to be used, on the other hand, it is generally preferable that the heater be located at the upper end of chamber 2% and that the tool extend a sufi'icient distance below the bottom of the heater to permit cooling of the plastic below its softening temperature. Thermocouples are preferably provided on or in the Wall of the tool to permit accurate temperature control. The apparatus is not restricted to the use of electrical heaters and may employ other heating means. Electrical heaters are preferred, however, because they can be readily controlled and do not require the circulation of liquids or gases in the borehole.

In utilizing the apparatus shown in the drawing, the tool is first lowered to the bottom of the borehole. The hole may have been drilled to the total depth but in most cases it will be preferred to case it at intervals and reduce the diameter after each casing installation. A similar practice is used in lining wells with steel pipe or casing. The walls of the borehole may have been cleaned or scratched to remove the drilling mud and other materials adhering thereto before the tool is lowered into place. This is often unneccessary, however, because the pressure employed to apply the plastic will force the mud into the formation sufficiently to permit good bonding between the plastic and the borehole wall. A liquid casting resin or similar plastic material which may or may not contain a filler is then pumped downwardly through hose or tubing 12 into the tool. Sufficient pressure to force the plastic outwardly through ports 17 into the annular space between the tool and the borehole wall with the desired pressure should be used. After pumping has started, electric heater 21 is turned on.

After the annular space between the tool and the borehole wall has been filled with plastic and the heater has reached the operating temperature, the tool is raised in the borehole at a predetermined rate. As the tool is raised, the plastic is forced from chamber 16 through ports 17 into the annular space under suficient pressure to invade the Walls of the borehole to an appreciable distance. It is generally preferred that the casing mate- 6 rial be forced into the wall an inch or more in order to insure good bonding. As pointed out earlier, this distance can be controlled by regulating the viscosity of the plastic, varying the amount and particle size of the filler used and controlling the pump pressure. A pressure gauge in the hose or tubing at the surface can be employed to indicate whether the annular space is continually filled with plastic. The upward speed of the tool can then be synchronized with the pump speed to maintain constant pressure in the annulus. A weight indicator may be employed at the surface in a similar manner. With tubing 12 suspended from the travelling block in the well or borehole rig, plastic is pumped downwardly until the annulus between the tool and the borehole wall is filled. Thereafter, the pressure of the plastic against the packer on the tool results in an upward thrust on the tool and tubing which reduces the Weight reading on the indicator. Raising the tool and pumping the plastic at the proper rates gives a constant reading on the indicator somewhat less than the static reading. A further means for insuring that the annulus is filled with plastic at all times is to install a pressure switch on packer 19. The pressure exerted on the packer by the plastic as it is squeezed into the formation will actuate the switch and result in a signal at the surface. The signal conductor may be strapped to the outside of the hose or tubing with the power cable to the heater. The conductor and cable can be separately reeled as the tool is lifted from the borehole.

As the plastic is forced into formation and the casing tool is raised in the borehole, a plastic lining is formed in the annular space between the tool and borehole wall. This lining is cured by the heat from heater 21 as the tool is raised. The pumping rate and the rate at which the tool is elevated should be controlled so that the plastic will harden sufiiciently to be self-supporting as the tool moves past it. As pointed out above, the length of the lower section of the tool containing the heater and the curing time of the plastic employed will in part determine the rate at which the tool can be raised. The outer surface of the tool as it moves upwardly smooths the plastic and results in a liner or casing having an even inner surface. Vent tube 25 in the tool permits the equalization of pressure above and below the apparatus and avoids swabbing action as the tool moves up the hole. The entire borehole wall can be cased in this manner.

The invention can be further illustrated by referring to the results of tests in which liquid plastics were employed to form borehole casing in situ in a manner similar to that described above.

In a first experiment, a 4% inch borehole 20 inches long was drilled into an octagonal block of Indiana limestone measuring 9 inches across each face and 24 inches long. After the hole had been drilled, the block was placed in a pressure chamber sealed around the block and fitted with a piston which permitted the application of fluids to the wall of the borehole under pressure. A steel liner 4%. inches in diameter was placed in the hole. A commercially marketed epoxy resin containing 15 parts per hundred of the catalyst recommended by the resin manufacturer was then forced into the annular space between the borehole wall and the liner under a pressure of pounds per square inch and allowed to harden. After removal of the lining, examination showed that the hole had a hard plastic lining between A and 7 inch thick. A second sample was then prepared in similar manner and had a lining between A and inch thick. In each case about an hour and fifteen minutes was required for the plastic to set completely. Removal of the liner after about an hour prevented it from sticking in the hole.

Each of the linings formed in the manner described above was tested by pumping water under high pressure into the space between the block and the Wall of the pressure chamber. It was found that pressures between 600 and 800 pounds per square inch caused the rock to fail in tension just outside the plastic lining. After the relatively thin linings thus became unsupported, they col-- lapsed in response to the pressure.

Additional linings were then formed in the manner described above, except that pressures of 1200 pounds. per square inch were used to force the liquid into the borehole wall and tests were carried out at both am bient temperature and at 130 F. The linings formed at ambient temperature penetrated the borehole wall a distance of about /2 inch, while those formed at higher temperatures penetrated an inch or more. Tests of the linings indicated that they were capable of withstanding; pressures between about 2500 and about 3500 pounds; per square inch, depending upon the lining thickness.

Two linings 1% inch thick were then formed as described above, in one case using a viscous mixture of com-- mercial epoxy resin and catalyst and in another using the same resin and catalyst containing 60% by weight of sandstone dust. It was found that the thick, unfilled lining cracked during curing but that the filled lining curedl evenly and withstood an external pressure of 4000 pounds per square inch, the maximum which could be exerted. with the equipment available, without change.

Tests designed to measure the setting period of a number of commercial epoxy resins and catalysts at various temperatures were also carried out by forcing the resins. into 1 /2 inch diameter limestone cores and observing the time required for the resin to solidify. Both dry cores and cores saturated with water were employed. It was found-that the setting time .could be varied from asv little as 15 minutes up to several hours by varying the catalyst content. Tests made at temperatures between the ambient temperature and 160 F. showed that the use of elevated temperatures greatly accelerates the setting time. The presence of water in the cores did not prevent invasion of the resin into the core or prevent successful bonding.

The results obtained in the tests described above demonstrate that liquid resins can be applied to the walls of boreholes and thereafter cured under heat and pressure to form casing capable of withstanding the temperatures and pressures found in oil wells and similar boreholes, The advantages of casing formed in this manner over conventional steel casing make its use attractive under a variety of conditions.

What is claimed is:

1. A process for easing a borehole penetrating a subterranean formation which comprises lowering a conduit having a casing tool connected to the lower end thereof into said borehole, said casing tool forming a confined annular space adjacent the borehole wall; introducing a liquid polymer that can be solidified at elevated temperature into said borehole through said conduit; injecting said polymer from said conduit into said confined annular space between the borehole wall and said casing tool under sufiicient pressure to force said polymer a substantial distance into the formation; and heating said casing tool to solidify said polymer while moving said conduit and tool axially within said borehole.

2. A process as defined by claim 1 wherein said polymer is en epoxy resin.

3. A process as defined by claim 1 wherein said polymer contains particles of an inert filler.

4. A process as defined by claim 1 wherein said polymer is injected into said confined space at a pressure from about to about 3000 pounds in excess of the formation pressure.

5. A process for casing a borehole penetrating a subterranean formation which comprises lowering a conduit fitted with a mandrel located near the lower end thereof into said borehole, said mandrel forming a confined annular space adjacent the borehole wall; introducing a liquid polymer which can be solidified at elevated temperature and contains particles of an inert filler material into said borehole through said conduit; passing said polymer into said confined space between the borehole wall and said mandrel at a pressure from about 100 to about 3000 pounds per square inch in excess of the formation pressure; heating said mandrel to a temperature sufiicient to solidify said polymer; and moving said mandrel and conduit upwardly in said borehole as said poliymer is passed into said confined space and solidi- 6. A process as defined by claim 5 wherein said polymer introduced into said borehole contains solid polymer particles which accelerate the solidification of said polymer.

7. A process as defined by claim 5 wherein said polymer is exposed to gamma radiation from a source within said borehole as said polymer solidifies.

References Cited in the file of this patent UNITED STATES PATENTS 2,237,313 Prutton Apr. 8, 1941 2,366,036 Leverett et al. Dec. 26, 1944 2,556,169 Crouch et al June 12, 1951 2,609,052 Kantzer Sept. 2, 1952 2,892,444 Perkins June 30, 1959 2,897,779 Perkins Aug. 4, 1959 2,944,298 Bernhardt et a1. July 12, 1960 2,986,212 Schultz May 30, 1961 FOREIGN PATENTS 380,451 Great Britain Aug. 24, 1932 

1. A PROCESS FOR CASING A BOREHOLE PENETRATING A SUBTERRANEAN FORMATION WHICH COMPRISES LOWERING A CONDUIT HAVING A CASING TOOL CONNECTED TO THE LOWER END THEREOF INTO SAID BOREHOLE, SAID CASING TOOL FORMING A CONFINED ANNULAR SPACE ADJACENT THE BOREHOLE WALL; INTRODUCING A LIQUID POLYMER THAT CAN BE SOLIDIFIED AT ELEVATED TEMPERATURE INTO SAID BOREHOLE THROUGH SAID CONDUIT; INJECTING SAID POLYMER FROM SAID CONDUIT INTO SAID CONFINED ANNULAR SPACE BETWEEN THE BOREHOLE WALL AND SAID CASING TOOL UNDER SUFFICIENT PRESSURE TO FORCE SAID POLYMER A SUBSTANTIAL DISTANCE INTO THE FORMATION; AND HEATING SAID CASING TOOL TO SOLIDIFY SAID POLYMER WHILE MOVING SAID CONDUIT AND TOOL AXIALLY WITHIN SAID BOREHOLE. 